The present invention relates to a process for reducing the sludge derived from a wastewater treatment unit.
Aerobic biological treatments of effluents generally consist in bringing these effluents into contact with a biomass (microorganisms) which degrades the pollution contained in said effluents by converting the organic molecules to inorganic molecules: this is the step commonly called aeration of the biological tanks. The use of such treatments results in a gradual increase in the amount of biomass and in the need to evacuate the excess biomass, commonly called “excess sludge”. Various solutions have been proposed in order to deal with the constantly increasing amount of this excess biological sludge and the evacuation thereof.
A first family of processes consists in removing this excess sludge at the end of the biological treatment and either finding it a suitable outlet or treating it in a specific degradation step. It can thus be used as fertilizer in agriculture (manuring). However, the adhering to environmental standards and the possible presence of micropollutants or of heavy metals in the sludge result in this use being decreased. Another solution consists in removing this sludge and incinerating it; it is then necessary to transport it to an incinerator, which involves some expense. In addition, the difficulties in installing new incinerators is hindering the development of this solution. Another solution consists in performing wet oxidation of the excess sludge: the sludge is then mineralized.
A second family of processes consists in reducing the production of sludge during the biological treatment. These solutions consist in using means for reducing the production of sludge during the biological process for depolluting water. These solutions consist in carrying out a partial lysis of the sludge, i.e. destroying some of the microorganisms which make up the sludge by making them partially soluble. The products derived from this lysis, which contain at least partially soluble organic compounds, can then be sent to the head of the effluent treatment in order to undergo the biological treatment, during which the microorganisms will treat the products derived from the lysis. A first known lysis technique consists in exercising a mechanical action on the sludge originating from the biological treatment tank, which causes some of the cells of the microorganisms constituting the excess sludge to rupture. This may be mechanical grinding, a compression/release technique, sonochemistry, etc. This technique is generally simple to carry out but has the disadvantage of only slightly reducing the excess sludge production. A second lysis technique is a basic or acid attack using chemical agents, optionally coupled with an increase in temperature, but this technique requires the pH of the solution obtained to be readjusted before it is reinjected into the aeration tank. The disadvantage of this solution is that it increases the salinity of the hydrolysed sludge, which can result in a dysfunction of the biological treatment step. A third lysis technique is based on the action of oxidizing agents such as: ozone, air, bleach, hydrogen peroxide or oxygen under pressure. The disadvantage of air, of hydrogen peroxide and of oxygen is that they are not effective enough alone: they must be combined with heating and/or a catalyst, which also increases the cost of these techniques. Document U.S. Pat. No. 4,269,714 describes a biological treatment process using micronization of the effluents and oxidation, providing both treatment of the sludge in a shorter amount of time and a reduction in the sludge produced. The micronization must be carried out in such a way as to avoid destruction of the microorganism cell walls, the only aim of the micronization being to increase the active surface of the flocs by separating the various constituents thereof.
The aim of the present invention is to provide a novel process for reducing the production of sludge derived from the treatment of wastewater.